1. Field of the Invention
The invention relates to a thermodynamic power cycle.
2. Description of the Related Art
In order to achieve high thermal efficiencies, conventional steam cycles require high operating pressures in addition to preheating of feed water before it begins absorbing heat from a heat source. With high operating pressures and preheating, one can obtain a high average temperature of heat absorption. However, both techniques have limitations which make it difficult to obtain high efficiencies.
Increasing pressure is limited by the maximum working temperature of the working fluid. If the working temperature is not high enough for a given pressure, water will condense in the turbine, reducing the isentropic efficiency thereof and increasing blade deterioration and maintenance cost. At a given maximum working temperature, which may be limited by, for example, corrosion problems, the heat source, economic reasons, etc., the only way to raise the pressure beyond the corresponding limit is by reheating the steam at an intermediate pressure. This process is costly and usually is not feasible in medium-size plants. Also, any pressure increase results in a corresponding decrease in the global efficiency of the turbine, partly due to the low specific volume of the steam.
The regenerative preheating of the feed water is disadvantageous because it must be accomplished by steam extractions from the turbine and thus its effectiveness is proportional to the number of these extractions. In smaller size plants, the number of steam extractions from the turbine is limited. Because of this limitation, as well as the complexity and cost of the cycle as a whole, cycle efficiency is reduced.
On the other hand, when one tries to utilize a secondary cycle of ammonia, the steam discharged by the steam cycle is wet or very close to saturation and, therefore, the ammonia can not be superheated unless steam is extracted from the turbine. This involves a great irreversibility and efficiency loss. The alternative of expanding the ammonia from the saturation line also diminishes the efficiency of the ammonia turbine and increases the maintenance cost.